Box Tube Trailer Frame

How would you go about straightening up the rear of this trailer? Frame is 2"x8" box. It is a 24' 12k trailer.

The 3" C channel is for a straight reference. It drops about 1.25" on each side right behind the axles.

Thanks

In our last shop we put in some heavy anchors pots. Think I could tie it down at the axels and jack the back but I would not have cut out the cross members. In the absence of anchors you could cross block like three cross ties there at the back three at the axels the place done across and take something like an excavator and apply pressure real slow. I am certain I could straighten the frame but it will always be weak where the bend was. will need to be reinforced with your long angel.

You might be better off to take it to a body shop that has a frame rack they can control the bend.

Weld an upright post (temporarily) at the point of the bend(s), then hook up a cone-a-long front and back ends and tighten till straight. When you get it straight see how much it springs back, and go that much tighter, when you tighten the second time.

Block up the rear and front of the frame on a very level concrete pad so that the middle is off the ground about 4 inches, so the wheels are not touching. Place a reference block under each side with a height that is effectively the height that would be straight. Use a one inch rose bud torch tip to get a triangle area 12" wide at the top and point down, to a medium red (both sides evenly heated) Heat at the most bent location. The heat will begin to sag the frame after making it appear to be worse at first.
Then quench the heat with a bucket and soaked rags cooling it as quickely as possible. This shrinks the steel and reduces the bend. Repeat this exactly the same on the other side. Repeat as needed to put the stringers right. some change in temper may be the result of this treatment, but not much. I recommend putting a 10 foot piece of that channel on the top of each side of each stringer, or if too close to the tires. Put a 1/4 inch plate (Fishplate) on that side matching the length. Skip weld 3 inches and leave 3 inches on the top and bottom of each.
That is the best I have, we did this to flat bed road trailers for steel hauling. 2-35,000# coils, 8 axles. They were bent the other direction, and we were putting arch into them from sagging 3 or 4 inches. Jim

Thanks for the suggestions so far! I'm trying to make myself live with it but is is getting lots of love right now (welder & new steel) and will be blasted and painted, so if I ever want it straightened, now is the time.

Crossmembers got cut out because they rusted out on one side anyway. This is my first and last box tube trailer!

The heat and cool method will be the best way to go about it. Anything else and it will just bend again. I would cut some off the back and shorten it as well. That is just not enough frame for that length trailer.

might be easer to cut off the shackles and fenders flip it over and weld it back together you will have a pre stressed frame. you said its bent the same on both sides 2CTS

I would re-inforce it with some 6" channel on the inside of the box tubing. Inside meaning under the floor. I wouldn't worry about 1.25".

You'll never be able to heat a big enough area to straighten it correctly. You'll most likely stretch the bottom and wrinkle the top if heating and bending it, then it'll be really weaker.

I had a 16' box tubing frame trailer that had the same problem. I never did anything to it and it lasted for 20 years. I sold it last year to buy a different trailer.

1 1/4" isn't much of a bend for something that long. I'd try something simple first. I used to build a lot of big oilfield skids. Everyone of them had to be straightened after welding. Put some weight on the trailer over the axles, maybe 1000 lbs. or more if you can. Block up the front corners of the trailer so the jack isn't taking all the weight. Then use a jack to lift the back of the trailer. The wheels about 3" inches off the ground. Then take your heating tip and heat up the top of the tubing where the bend starts. Looks like a few inches back of the angle for the fenders. Only heat the top of the tubing but make sure the corners of the tubing get hot about 3/8" down the sides. The corners are where most of strength is so they have to be hot. I think you'd only need to heat it about 1-1 1/2" wide at the most. Use some soap stone to make lines in the same place on each frame rail. I'd stand in the middle and heat it evenly on both sides till it starts to glow. You don't want/need to heat it where it's almost ready to melt. Then just let it cool off on it's own with the jack holding it up. As the heated area cools it should contract and pull it straight. I wouldn't use water to cool it because it's such a minor bend. Once it cools to room temperature, take the jack off and check it for straightness. If it needs to go more you can always heat it again and maybe use a wet rag to help it contract but the first try I'd see if it will pull without using water. Heating the top to let it contract is better then heating the bottom and trying to stretch it.

To reinforce it after it's straight, I'd just stitch weld some 1/4" fish plates on the inside of the frame about a foot long or so. Only weld the fish plates along the top and bottom and not vertical on the 8" side. Channel would be overkill for reinforcing because the tubing frame already holds it from twisting in other directions. All you want to do is help prevent it from bending down again. There's no concerns about it bending from the side or any other direction. When you weld the new cross members in stagger your welding and do the vertical welds first. Weld the top and bottom of the cross members last.

Put another axle back farther to accomadate the over hanger

bingo, right on!
that's what I would do. then reinforce it.
good luck
Ken

He has torflex axles so he can't do that.